A category A substance (pathogen or agent) is “an infectious substance which is transported in a form that, when exposure to it occurs, is capable of causing permanent disability, or life- threatening or fatal disease to otherwise healthy humans or animals”.

Deciding if an infectious substance is a Category A substance is relatively easy because there are relatively few Category A substances and because Category A substances are specifically designated and listed by IATA and DOT. The list of Category A substances is not all-inclusive, and a thorough risk assessment must be performed before assigning a substance to Category A.